(a) Field of the Invention
The present invention relates to a semiconductor light emitting device, more in detail, to the semiconductor light emitting device in which an optical output and a lasing spectrum have a stable relation with respect to injected current, and more specifically to the high power semiconductor pumping source, such as a semiconductor laser, which is most suitable as an optical amplifier for an optical communication system.
(b) Description of the Related Art
A semiconductor light emitting device outputting a light having a wavelength of a 980 nm band, for example, an InGaAs-based semiconductor laser element is frequently employed as various light sources in an optical communication system, for example, as a pumping light source for an optical fiber amplifier.
The structure of the conventional InGaAs-based laser element of the 980 nm range (hereinafter referred to as xe2x80x9cInGaAs-based laser elementxe2x80x9d) will be described referring to FIG. 1.
A conventional InGaAs-based laser element 20 basically has a layered structure shown in FIG. 1 which is manufactured by sequentially depositing, by means of epitaxial growth, a n-AlGaAs cladding layer 12 having a thickness of 2 xcexcm, quantum well active layers 13 including InGaAs films having a thickness of 7 nm and GaAs films having a thickness of 10 nm, a p-AlGaAs cladding layer 14 having a thickness of 2 xcexcm and a GaAs cap layer 15 having a thickness of 0.3 xcexcm, on an n-GaAs substrate 11 having a thickness of 100 xcexcm.
The top portion of the cladding layer 14 and the cap layer 15 are subjected to an etching to form a mesa-stripe. A passivation film 16 made of SiN is formed on the entire surface other than the top surface of the cap layer 15 to make a window for supplying current therethrough.
A TiPtAu metal layered film 17 is formed on the passivation film 16 and the top surface of the cap layer 15 as a p-side electrode, and an AuGeNiAu metal layered film 18 is formed on the bottom surface of the substrate 11 as an n-side electrode.
Then, a conventional method for manufacturing the InGaAs-based laser element 20 shown in FIG. 1 will be described referring to FIG. 2 showing a vertical section of the laser element 20 during the manufacture.
At first, the n-AlGaAs cladding layer 12 having a thickness of 2 xcexcm, the quantum well active layers 13 including InGaAs films having a thickness of 7 nm and GaAs films having a thickness of 10 nm, the p-AlGaAs cladding layer 14 having a thickness of 2 xcexcm and the GaAs cap layer 15 having a thickness of 0.3 xcexcm are sequentially formed on the n-GaAs substrate 11 by using an MOCVD method.
Then, the cap layer 15 and the upper portion of the cladding layer 14 are selectively etched by a thickness of 1 xcexcm to form a mesa stripe, followed by formation of the passivation film 16 on the entire surface of the mesa stripe. The passivation film on the top surface of the mesa stripe is removed to form a window through which current is to be injected.
Then, the bottom surface of the substrate 11 is polished until the thickness of the substrate becomes 100 xcexcm. Thereafter, the TiPtAu metal layered film 17 functioning as a p-side electrode and the AuGe/Ni/Au metal layered film 18 functioning as a n-side electrode are formed on the passivation film 16 and on the bottom surface of the substrate 11, respectively, by means of vapor deposition to provide the InGaAs-based laser element 20.
Since the bandgap energy Eg1 of the substrate 11 of the conventional InGaAs-based laser element 20 is 1.41 electron volts (eV) and the bandgap energy Eg2 of the active layers 13 is 1.27 electron volts which is smaller than the bandgap energy Eg1, the light emitted from the active layer can travel within the substrate.
However, the conventional InGaAs-based laser element has the following two problems in connection with the characteristics of the laser element.
A first problem is that a linearity between current and an optical output is not excellent and kinks are observed in the current-optical output characteristic as shown in FIG. 3 wherein the wavelength and the optical output power are plotted on abscissa and ordinate, respectively. The disturbance on the current-optical output characteristic makes it difficult to obtain a stable automatic power control (APC) for the InGaAs-based laser element based on the current-optical output characteristic.
A second problem is that a ripple appears at a specified wavelength interval, for example, a 3 nm interval on the wavelength characteristic of a lasing spectrum as shown FIG. 4. In this situation, a phenomenon occurs in which the lasing mode skips every 3 nm interval with respect to the injected current, and mode hopping noise is generated.
Accordingly, it is difficult to amplify the input signals simultaneously to keep the signal powers constant using, the conventional InGaAs-based laser element used for pumping source in an optical amplifier.
This disadvantage of the semiconductor light emitting device is extremely undesirable used for the pumping source of an optical fiber telecommunication system.
The present inventors have found the following matters after investigating the reasons of causing the above problems of the InGaAs-based laser element.
Either problem is generated because light is regularly reflected on the bottom surface of the substrate. Since the bottom surface of the substrate is of a mirror surface after the substrate is polished at the bottom surface to have a specified thickness, for example, to a thickness of 100 xcexcm, part of the light travelling within the substrate and reflected on the bottom surface of the substrate is coupled with the light in the active layer 13 as shown in FIG. 5, and a coupled cavity is formed by combination of a substrate resonator formed by the regular reflection on the substrate and an original Fabry-Perot resonator, to thereby make the lasing wavelength unstable.
In view of the foregoing, an object of the present invention is to provide a semiconductor light emitting device such as a semiconductor laser element which can obtain a multiple longitudinal mode lasing spectrum generating reduced ripples and a stable optical output with respect to injected current.
The present invention provides a semiconductor light emitting device including a semiconductor substrate having a bandgap energy of Eg1, and at least one active layer having a bandgap energy of Eg2 smaller than Eg1 and overlying main surface of the semiconductor substrate, a bottom surface of the semiconductor substrate opposite to the main surface having unevenness for diffused reflection.
In accordance with the present invention, the generation of kinks in the current-optical output characteristic and of ripples in the lasing spectrum occurring in a conventional semiconductor light emitting device can be substantially suppressed. Further, a semiconductor light emitting device can be provided in which an optical output and a multiple longitudinal mode lasing spectrum stably operate with regard to injected current by reducing an amount of light reflecting from the semiconductor substrate bottom surface to the active layer by means of making the minute unevenness on the semiconductor substrate bottom surface. Accordingly, the most appropriate high power pumping source for an optical amplifier of an optical fiber telecommunication system can be obtained.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.